The procedures employed in the production of corrugated paperboard generally involve a continuous process wherein one strip of paperboard is fluted by means of heated, fluted rollers. The protruding tips on one side of this fluted paperboard strip are then coated with an adhesive, and a flat sheet of paperboard, commonly referred to in the trade as a facing, is brought into contact with these adhesive-coated tips. By applying heat and pressure, an adhesive bond is formed there between. This process produces what is known as single-faced board, named for the fact that the facing is applied to only one surface. If a double-faced paperboard (in which an inner fluted layer is sandwiched between two facings) is needed, a subsequent operation is performed wherein the adhesive is applied to the exposed tips of the single-faced board and the adhesive-coated tips are then joined with a second facing under pressure and heat. Thus, both single and double-faced board can be produced in a continuous process. The typical use of these types of adhesives in a corrugating process and the use and operation of corrugators in general are described in U.S. Pat. Nos. 2,102,937 and 2,051,025 to Bauer.
A major concern in such processes is the selection of the appropriate adhesive, as its properties will affect not only the strength and stability of the final corrugated product, but also the parameters (corrugator speed, etc.) within which the process can be operated. Thus, such adhesives are chosen in light of the specific requirements of the process and the properties desired in the ultimate corrugated paperboard. The adhesives most commonly used in corrugating are starch-based adhesives which are popular because of their desirable adhesive properties, low cost, and ease of preparation.
The most common type of starch-based corrugating adhesives comprises the alkaline adhesives. Such adhesives are comprised of dried raw, ungelatinized starch suspended in an aqueous dispersion of gelatinized (cooked) starch termed the carrier. These adhesives are produced by gelatinizing starch in water with sodium hydroxide (caustic soda) to yield a gelatinized (or cooked) carrier which is then blended or mixed with the raw (ungelatinized) starch, borax and water to produce the adhesive.
In the corrugating process, the adhesive is applied (usually at a temperature between 25.degree. and 55.degree. C.) directly to the tips of the fluted paper medium and the facing is brought into contact with the adhesive-coated surfaces; the subsequent application of heat causes the raw starch to gelatinize, resulting in an instantaneous increase in viscosity and tack, thereby forming of the adhesive bond between the fluted paper and the facing.
For various applications, the adhesive composition can be varied to maximize one or more desirable properties such as extended pot life and viscosity stability, water resistance of the ultimate bond, etc. However, one property directly related to the utility of the formulation as a corrugating adhesive is its ability to rapidly form an initial bond (termed "tack" or "green bond strength"); this rate of tack development is directly related to the speed at which the corrugator can be run. There exists a need for corrugating adhesives possessing high green bond strengths and rapid development of tack.
While there are different theories regarding the respective roles of the raw starch and the carrier in the development of adhesive properties, some research has centered on the view that the carrier contributes to the bond strength and setting speed of the adhesive; indeed, it has been suggested that good tack in the carrier leads to good tack (and therefore improved runnability) in the adhesive (see e.g., R. Williams, C. Leake and M. Silano, TAPPI, Vol. 60, No. 4 April/1977 pp 86-89). Further, it has been known for many years that a corrugating adhesive whose carrier portion is prepared from a high amylose starch exhibits superior tack to one prepared from pearl starch, which contains about 27% amylose, because the high amylose carrier will have improved rheological and film-forming properties, and increased moisture resistance. Thus, there are many different carriers available for use in starch based adhesives.
To date, however, relatively little experimentation has been done with the raw (uncooked or ungelatinized) starch portion of the adhesive, other than varying the species of starch source utilized. Generally, the raw starch is supplied in dried powder form, and is used as such in the adhesive preparation. Thus, during commercial processing the starch is ordinarily dried in an elevated temperature dryer (generally at 200.degree.-250.degree. F., 94.degree.-112.degree. C.) (to remove water to about 8-14% moisture content), after which it is packaged and stored until use.